Photomasks are used to form an image to be projected onto a silicon wafer treated with photoresist. The photomask typically has a light blocking and transmitting pattern. The photomask may be formed by patterning a mask formed of a metal, such as chrome, that was deposited on a substrate, such as quartz. The patterned substrate, often called a "reticle," may be used to define an image which is projected onto the semiconductor wafer. The photoresist covering the wafer is thereby exposed in the desired pattern. After the pattern is developed, the developed photoresist may be used to define structure such that the original pattern is reproduced in that structure on the semiconductor wafer.
Typically in wafer lithographic systems, a light source is projected through the reticle onto the wafer. The wafer is positioned under control of a device called a stepper. The stepper aligns the reticle to the wafer.
There is a continuing need to improve photolithographic techniques to enable smaller integrated circuits to be fabricated. As smaller integrated circuits are possible, their performance, size and cost may be improved. Successive generations of lithographic techniques used for forming such images have provided increasingly improved resolution and enabled ever smaller devices to be made.
However, these lithographic systems are expensive. Thus, it would be desirable to improve the resolution achievable with a given system without requiring the substantial capital investment to replace and upgrade the equipment.
As light passes through the photomask, some of the light is diffracted at the interface between the masking and nonmasking areas of the pattern. The diffracted light decreases the image resolution of the projected image. As a result, the projected image and the structures formed on the semiconductor wafer may be of diminished quality. Ultimately, the size of the smallest image which can be transferred to the wafer is reduced as a result of this diffraction effect.